Targeting miR-21 with NL101 blocks c-Myc/Mxd1 loop and inhibits the growth of B cell lymphoma

Background: NL101 has shown activities against multiple myeloma and acute myeloid leukemia, but its anti-lymphoma activity remains unknown. The transcription factor c-Myc is frequently dysregulated in aggressive B cell lymphomas such as double-hit lymphoma, for which the standard of care is still lacking. A novel approach to target c-Myc needs to be explored. Although the role of oncogenic microRNA-21 (miR-21) was well established in an inducible mice model of B cell lymphoma, whether targeting miR-21 could inhibit the growth of B cell lymphoma and its underlying mechanisms is unclear.

Methods: We used MTT assay and flow cytometry to determine the inhibitory effect of NL101 on the cell proliferation of B cell lymphoma in vitro. The lymphoma xenograft mice models were generated to evaluate the anti-lymphoma function in vivo. Western blot and qPCR were applied to measure the expression levels of protein and microRNA, respectively. To investigate the mechanisms of action in NL101, we used genechip to profile differentially-expressed genes upon NL101 induction. Luciferase reporter system and chromatin immunoprecipitation were used for the validation of target gene or miRNA.

Results: Nl101 significantly inhibited B cell lymphoma proliferation through induction of cell cycle arrest and apoptosis. NL101 suppressed the growth of B cell lymphoma in vivo and prolonged the survival of lymphoma xenograft models. Gene expression profiling revealed that miR-21 was significantly decreased upon the induction of NL101 in B cell lymphoma. The miR-21 level was associated with the sensitivity of NL101. miR-21 inhibited Mxd1 expression via directly combining to Mxd1 3'-UTR; c-Myc activated miR-21 expression by directly binding to the miR-21 promoter.

Conclusion: NL101 significantly inhibited the growth of B cell lymphoma in vitro and in vivo. The novel c-Myc/miR-21/Mxd1 positive-feedback loop is critical for the maintenance of B cell lymphoma survival. Targeting miR-21 to block c-Myc/miR-21/Mxd1 loop represents a novel potential strategy of c-Myc-directed therapy.

Mxd1 mediates hypoxia-induced cisplatin resistance in osteosarcoma cells by repression of the PTEN tumor suppressor gene

Hypoxia-induced chemoresistance remains a major obstacle to treating osteosarcoma effectively. Mxd1, a member of the Myc/Max/Mxd family, was shown to be involved in the development of drug resistance under hypoxia. However, the effect of Mxd1 on hypoxia-induced cisplatin (CDDP) resistance and its mechanism in osteosarcoma have not been fully elucidated. In this study, we demonstrated that HIF-1α-induced Mxd1 contributed to CDDP resistance in hypoxic U-2OS and MG-63 cells. The knockdown of Mxd1 expression elevated PTEN expression at both protein and RNA levels in these hypoxic cells. Using Luciferase reporter and ChIP assays, we confirmed that Mxd1 directly bound to the E-box sites within the PTEN promoter region. We further demonstrated that PTEN knockdown decreased CDDP sensitivity in Mxd1 siRNA-transfected U-2OS and MG-63 cells under hypoxia. Our results also showed that Mxd1 deficiency in hypoxic U-2OS and MG-63 cells lead to inactivation of PI3K/AKT signaling, which is the downstream of PTEN. Furthermore, blockade of PI3K/AKT signal re-sensitized hypoxic U-2OS and MG-63 cells to CDDP. Taken together, these findings suggest that HIF-1α-induced Mxd1 up-regulation suppresses the expression of PTEN under hypoxia, which leads to the activation of PI3K/AKT antiapoptotic and survival pathway. As a result CDDP resistance in osteosarcoma cells is induced.

Id2 deletion attenuates Apc-deficient ileal tumor formation

The expression level of inhibitor of DNA binding 2 (Id2) is increased in colorectal carcinomas and is positively correlated with poor prognosis. However, the functional significance of Id2 in intestinal tumorigenesis has not been fully defined using genetic approaches. Here, we show that Id2 promotes ileal tumor initiation in Apc-deficient mice. Expression of Id2 was stimulated by Wnt signaling through the enhancer region of the Id2 promoter at the early stage of tumorigenesis in Apc^+/Δ716 (Apc^Δ716) mice. Genetic depletion of Id2 in Apc^Δ716 mice caused ∼80% reduction in the number of ileal polyps, but had little effect on tumor size. Notably, the lack of Id2 increased the number of apoptotic cells in the normal crypt epithelium of the mice. Furthermore, DNA microarray analysis revealed that the expression level of Max dimerization protein 1 (Mxd1), known as a c-Myc antagonist, was specifically increased by Id2 deletion in the ileal intestinal epithelium of Apc^Δ716 mice. In contrast, the protein level of c-Myc, but not the mRNA level, was decreased by loss of Id2 in these mice. These results indicate that loss of Id2 inhibits tumor initiation by up-regulation of Mxd1 and down-regulation of c-Myc in Apc^Δ716 mice.